On at least some early examples, the band is painted in Sky Blue. Some people deny this, but it was certainly in a light colour contrasting with the Sky undersides, and appearing to look like Sky Blue in photos.

The aircraft spinner is normally painted in the same colour as the band.

From very late in 1945, this band was painted out on aircraft in 2 TAF on the continent.

It was not carried on fighter aircraft overseas, until very late in the war when it was seen in Italy. It was not normally carried by the Fleet Air Arm but sometimes seen on units that on formation had spent some time in the UK.

Can I suggest that if you thought this band was white then there may be other aspects of RAF camouflage and markings where you may benefit from the knowledge of this board. Perhaps you could describe your project in a little more detail?

My Apologies for the late comeback on this post chaps, all your comments were helpful and appreciated.

Graham, the book I am working on is still `Aircraft losses around the Scottish Isles` but not a complete listing of all crashes, but selected detailed accounts on various types lost between 1914-1941 in the first volume, accounts of around 50 aircraft, with mini-bios on many of the crew members. This is a Co-Write with Peter Dobson, some of whom will know through his research of Cumbrian aircraft accidents and the former Cumbrian Aviation Research Society. It was our first intention to have the book in colour, but rising costs force us to adopt B/W format, so where colour profiles would have been used, it seems not as critical now, though I would still like to get sq markings and code letters correct, though most of the aircraft are Coastal Command, RN FAA or Luftwaffe up to 1941.

So in effect with regards to that band on the rear fuselage, whether it be sky blue or white, in our now adopted format it will appear white anyhow.

Mind you, there were differences in style of applied codes between various squadrons, and sometimes within a squadron itself. Also finish of aircraft could vary, and to be on the safe side, you should need a photo evidence of the aircraft to be portrayed.
I doubt different colours were used for Sky bands. I would rather expect the difference caused by different age of apllied layer, different method of applying the band (brush, broom, spray), different manufacturers or different batches of paint, etc.
The band was introduced to avoid friendly fire incidents and improve air to air recognition, it was remo0ved or not applied where it could compromise the aircraft on the ground.